As for an interior permanent magnet motor (IPM), whose permanent magnets are arranged in the interior of the motor, the resultant electromagnetic torque includes a permanent magnet torque and a reluctance torque. The permanent magnetic torque is generated by the rotor permanent magnetic field interacted with the stator magnetic field. The reluctance torque is generated by the stator magnetic field interacted with the rotor iron core whose direct-axis inductance and quadrature-axis inductance are different. As compared with a surface-mounted permanent magnet motor (SPM), whose permanent magnets are mounted on the surface of the rotor iron core, the interior permanent magnet motor (IPM) can achieve the high efficiency more easily with the increased utilization of the reluctance torque.
Whereas the permanent magnet assisted synchronous reluctance motor with multiple layers of permanent magnets arranged in the rotor, as compared with the IPM motor, further increases the difference between the direct-axis inductance and the quadrature-axis inductance, greatly increases the utilization of the reluctance torque, realizes the high efficiency of the motor, and greatly reduces the cost of the motor. For example, the patent ZL201210056204.8 discloses a rotor structure capable of increasing the utilization of the reluctance torque of a motor.
But the research found that, along with the increase of the utilization of the reluctance torque of a motor, the percentage of the reluctance torque in the resultant electromagnetic torque would be increased, and thus increasing the torque ripple of the motor, and further causing problems of vibrations and acoustic noises of the motor. Especially when a field weakening control for the motor is needed in order that the motor can run at a high speed, the percentage of the reluctance torque will be greater, and the problems will be more significant.
US patent application with publication No. US20100079026 discloses a permanent magnet motor, whose number of stator slots per pole pair is an odd, wherein, through adjusting the angle profile of the rotor permanent magnets, the magnetic field profile of the permanent magnets is improved, thereby reducing the harmonic content, further reducing the eddy current loss and the torque ripple. According to the patent application, what improved is mainly the magnetic field profile of the rotor permanent magnets, which helps to reduce the permanent magnetic torque ripple, but has no effect on restraining the reluctance torque ripple. What's more, as the number of stator slots per pole pair is an odd, it means that the number of stator slots per pole per phase (the number of stator slots/the number of pole pairs/the number of phases of windings/2) is a fraction. The current research shows that, the stator magnetic field of the motor whose number of the stator slots per pole per phase is a fraction, has much more harmonic content than the stator magnetic field of the motor whose number of the stator slots per pole per phase is an integer, therefore, the former motor is not good for reducing the reluctance torque ripple.
As compared with the motor with a single layer of permanent magnet, the permanent magnet assisted synchronous reluctance motor with multiple layers of permanent magnets arranged in the rotor may increase the difference between the direct-axis inductance and the quadrature-axis inductance, thereby greatly increasing the utilization of the reluctance torque, and realizing greater output torque and higher efficiency; but some problems will occur along with the increase of the percentage of the reluctance torque in the total electromagnetic torque due to the main reason that, the reluctance torque is apt to generate ripples due to the change of the relative positions of the stator and the rotor, which will increase the electromagnetic torque ripple of the motor, thereby causing the motor to vibrate and generate larger noises.
In the prior art, in order to reduce the torque ripple of the permanent magnet assisted synchronous reluctance motor, a common method is increasing the number of the stator slots of the motor and the number of the layers of the permanent magnets arranged in the rotor, but this method will make the manufacturability of the motor more complex, and the reduction extent of the torque ripple is limited. As shown in FIG. 1, the permanent magnet assisted synchronous reluctance motor comprises a stator 1 and rotor 4. Wherein, the stator comprises at least the stator iron core 2 made of magnetic material and the stator windings 3; the rotor 4 comprises at least the rotor iron core 5, each rotor pole of the rotor iron core comprises multiple permanent magnet grooves 6a and 6b, and permanent magnets 7a and 7b arranged in the permanent magnet grooves.